IE911626A1 - Lightweight aggregates for reduced-density concretes and method for obtaining them - Google Patents

Abstract

The granulate, such as vermiculite, is treated by spraying the surface of the particles with the help of a mixture of three adjuvants, namely a surface-active agent, a setting accelerator and a cement-fluidising agent, as a mixture in water, and the mixer for the preparation of the granulate comprises a rotary central shaft 7 passing through a stationary feed hopper for particles 1, carrying a perforated rotating tray 3 through which passes a liquid feed conduit 6 ending in a spraying nozzle 8. <IMAGE>

Description

LIGHTWEIGHT AGGREGATES FOR REDUCED-DENSITY CONCRETES AND METHOD AND APPARATUS FOR OBTAINING THEM The present invention relates to lightweight artificial or non-artificial aggregates with increased density-reduction capabilities for producing non-load bearing, reduced-density concretes. It also relates to a method for preparing coated aggregates and to blending apparatus for obtaining such coated aggregates.

In the production of reduced-density concrete, the use is known of lightweight aggregates such as expanded schistes and clays, pumices, pozzolana, perlites and vermiculites, expended polystyrenes, wood chips and cork.

Such aggregates, which have a density of less than 1, are preferable pre-calibrated by eliminating the fine fraction, for example those particles less than 1.4 mm in diameter, for avoiding a too large amount of additive being absorbed which would be prejudicial to the desired characterls20 tics of the material.

Such aggregates are principally characterized by a high porosity with respect to conventional heavy aggregates. High porosity leads to undesirable properties in the production of a lightweight concrete which is compact and has good mechan25 ical strength. In effect, high porosity provides the lightweight aggregate with a high capacity to absorb water, thus making it necessary to use large amounts of water for correctly hydrating the cement and obtaining concrete that has good workability.

The use of large amounts of make-up water introduces in its turn other disadvantages such as a reduction in mechanical characteristics and an increase in the setting and drying time of the concrete along with poor workability of the concrete and a danger of the cement segregating out.

The present invention enable lightweight aggregates to 5 be obtained which have an improved density-reducing capacity for non-load-bearing concrete, and of which the amount used per meter cubed of concrete is reduced, thus enabling the amount of cement used to be increased without leading to a variation in the density of the hardened concrete, along with better workability of the fresh concrete for a reduced makeup water:cement ratio.

Such a result is obtained through the synergic effect of several additives which are absorbed onto the surfaces of the aggregates.

The aggregates according to the invention are characterized by the fact that their surface is coated with a mixture containing a surfactant, a cement set accelerator and a cement liquefying agent.

According to one embodiment of the invention, the surfactant is preferably a synthetic anionic surfactant. It can, among other substances consist of at least one substance from the following list which should not be considered as limiting: - sodium abietate, - ethanolamine and triethanolamine salts, - fatty alcohol sulfonates, - alkylarylsulfonates, - sodium soaps of polyhydroxycarboxylic acids, - alkaline soaps of to c^g carbon chain natural fatty acids, - surfactant derivatives of a protein nature based on animal blood.

This surfactant leads to air being released at the time the concrete is being mixed and contributes, firstly, to reducing the density of the concrete, an improvement in its compactness through the fine occluded air bubbles that play the role of a fine sand filling the interstitial spaces and, secondly, to a considerable improvement in the workability of •E 911626 the concrete right up to a point where it can be pumped. This amount of air is controlled both by the amount of make-water used and by the mixing time which is notably determined as a function of the speed of the mixing device.

In accordance with a further feature of the invention, the cement set accelerator is preferably sodium nitrate and/or chloride. It can also consist in at least one compound from the following list: - sodium chloride, - sodium or potassium hydroxide, - alkaline-based salts such as carbonates, silicates, fluosilicates, aluminates, borates, - calcium nitrite, - oxalic acid.

Any other known accelerator able to reinforce the early mechanical characteristics can be employed. In accordance with a further feature of the invention, the cement fluidifying agent is preferably a polynaphtalene sulfonate with 40% dry matter content or any other known agent for this applica20 tion preferably having a high molecular weight, such as among other substances: - sodium lignosulfonate, calcium lignosulfonate, - formol melamin resin, - sodium abietate, potassium abietate, - resin soaps.

The essential function of this agent is to improve the plasticity and mechanical characteristics of the concrete while simultaneously reducing the amount of make-up water.

The invention also relates to a method for preparing 30 aggregates the surface of which is coated by absorption of the specific mixture of the above-stated additives, according to which the aggregates are treated in the presence of this mixture of additives and of water in a blending device.

In accordance with one feature of the method of the 35 invention, it consists in using for the treatment of 1 000 volumes of aggregate, a mixture of the three additives comprising surfactant, fluidifying agent and set accelerator in amounts of 1 000:1 000:4 000.

The invention also provides a blending apparatus for carrying out the method comprising a central rotatable shaft passing through a fixed particle feed hopper, said shaft carrying a perforated rotating plate and having a liquid feed conduit passing therethrough, said conduit terminating at a spraying nozzle, said apparatus further including a spray chamber into which said spray nozzle discharges.

Other features of the invention will become more clear from the description that follows and the manners of carrying out the invention which are provided by way of non-limiting illustration.

The blending device illustrated in the attached figure carries a feed hopper 1 at its upper end through which a rotatable shaft 7 passes, the axis of which includes a liquid feed conduit 6. The rotatable shaft 7 if integral with a perforated plate 3. The outer wall of hopper 1 is provided with deflecting vanes 2.

The lower end of conduit 6 is fitted with a spray jet 8 housed inside a spray chamber 5. Between the perforated plate 3 and the spray chamber 5 a funnel-shaped distributing element 4 is provided which reduces the diameter of the flow leaving the perforations of plate 3 and brings it down to the dimensions of the spray chamber. Chamber 12, the inner wall of which is fitted with counter-vanes 13, is driven in rota25 tion by means of a motor.

EXAMPLE 1 Preparation of an aggregate in accordance with the invention Vermiculite having the following characteristics was employed: . apparent bulk density ... about 60 kg/m . particle size ............ 6 to 1.4 mm The three additives were mixed in advance in a defined amount of water. Exfoliated vermiculite was introduced into the blending device through feed hopper 1 which sprayed it evenly over rotatable plate 3. This plate is perforated in order to increase the swirling motion of the particles that are already moving. The flow is then channeled by the fixed funnel-shaped member 4 prior to reaching spray chamber 5.

The liquid mixture of additives and water was fed under pressure through conduit 6 and sprayed through jet 8 into the heart of the fine descending swirling cloud of vermiculite. The swirling motion imparted to the particles particularly favours good contact between the sprayed liquid and the latter.

The surface-treated vermiculite was then collected in chamber 12 and supplied to the outlet opening 15 with the aid of a scraper 16.

The three additives were sprayed onto 1 000 liters of vermiculite in the following proportion: . anionic surfactant with 30% dry extract content .... 1 000 1 . fluidifying agent (naphtalene sulfonate) with 40% dry extract content ....................... 1 000 1 . set accelerator (sodium nitrate/sodium chloride in 50/50 weight percent proportion) ............... 4 000 g . water ..............................................34 000 g EXAMPLES 2 to 4 The thus-coated vermiculite was use in the preparation of concrete having the characteristics listed below.

Example 2 corresponds to a control using a vermiculite which had not being treated in accordance with the invention.

EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 Composition Untreated Treated Treated per m3 of Vermiculite - 1350 1 Vermiculite - 1000 1 Vermiculite - 1000 1 applied concrete Cement CP 545 - 260 kg Cement CP 545 - 300 kg Cement CP 545 - 200 kg Make-up water - 370 kg Make-up water - 240 kg Make-up water - 220 kg Water/cement ratio 1.423 0.800 1.1 Dry concrete density ’ 450 kg/m3 450 kg/m3 ’ 320 kg/m3 Workability Appearance of concrete Compressive strength after 7 days dry concrete fluid concrete fluid concrete porous compact compact '0.4 MPa 0.4 MPa *C - 50% HR it 911626 As can be seen from the table, the invention provides the advantage of making it possible to provide, through the use of the thus-treated aggregates, lightweight concretes which were improved compared to those obtained using untreat5 ed vermiculite.

For equivalent densities, the amount of make-up water was reduced and the rheological characteristics of the fresh concrete together with the mechanical performance of the hardened concrete were distinctly better.

The invention is obviously not limited to the embodiments that have been described and illustrated but may be subject to numerous variation available to those skilled in the art without this leading to a departure from the scope of the invention.

Claims (16)

CLAIMS:

1. - Lightweight aggregate material with an increased density-reduction capacity for use in producing non-loadbearing lightweight concretes, the surface of said aggregates 5 being coated with a mixture containing a surfactant, a cement set accelerator and a cement fluidifying agent.

2. - Granulate according to claim 1, wherein it consists of a material selected from the group comprising expanded schistes and clays, pumices, pozzolana, perlites and ver10 miculites, expanded polystyrenes, wood chips and cork.

3. - Granulate according to claim 1 or 2, wherein the aggregate of density less than 1 are pre-calibrated by elimination of the fine fraction having a size less than 1.4 mm.

4. - Granulate according to claim 2, wherein the aggreg15 ate material consists of vermiculite.

5. - Granulate according to any one of claims 1 to 4, wherein the surfactant is a synthetic anionic surfactant.

6. - Granulate according to any one of claims 1 to 5, wherein the cement set accelerator is sodium nitrate and/or 20 chloride.

7. - Granulate according to any one of claims 1 to 6, wherein the cement fluidifying agent is a polynaphtalene sulfonate with 40% dry extract content and high molecular weight. 25

8.- Method for preparing aggregates according to any one of claims 1 to 7, wherein a mixture of the above-defined additives and water is sprayed onto the aggregate in a blending device.

9. - Method according to claim 8, wherein it consists in 30 using for the treatment of 1 000 volumes of aggregate, a mixture of the three additives comprising surfactant, fluidify ing agent and set accelerator in amounts of 1 000:1 000:4 000.

10. - Blending apparatus for carrying out the method according to claim 8 or 9 comprising a central rotatable shaft 35 passing through a fixed particle feed hopper, said shaft carrying a perforated rotating plate and having a liquid feed conduit passing therethrough, said conduit terminating at a spraying nozzle, said apparatus further including a spray chamber into which said spray nozzle discharges.

11.- Blending apparatus according to claim 10, wherein a funnel-shaped distributing device is situated below said 5 rotatable plate having an upper inlet diameter which corresponds to the diameter of said perforated plate and the lower outlet diameter of which is located just above said spray chamber and has a diameter corresponding to the diameter of said spray chamber.

12. Lightweight aggregate material with an increased density-reduction capacity for use in producing non-load bearing lightweight concretes substantially as described in Example 1.

13. Method for preparing aggregates according to Claim 8 substantially as herein described.

14. Blending apparatus according to Claim 10 substantially 10 as herein described with reference to and as shown in the accompanying drawing.

15. Concrete comprising lightweight aggregate material according to any one of Claims 1 to 7 or 17 and 15 substantially as described in Examples 3 & 4.

16. The features described in the foregoing specification, or any obvious equivalent thereof, in any novel selection.